The elements of materia medica and therapeutics Plumbi carbonas. - Carbonate of lead.

The elements of materia medica and therapeutics
by Jonathan Pereira, M.D. F.R.S. & L.S.
Fourth Edition, enlarged and improved, including notices of themost of the medicinal substances in use in the civilized world, and forming an Encyclopædia of Materia Medica.
Vol. I.
London: printed for Longman, Brown, Green, and Longmans, Paternoster Row.
1854.

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¹ De Morbis, lib. ii.

² Theophrastus's History of Stones, by Sir J. Hill, 2d edit. p. 228, Lond. 1774.

³ Lib. v. cap. ciii.

⁴ Hist. Nat. lib. xxxiv.Formula PbO,CO₂. Equivalent Weight 134.

History. — Ceruse (cerussa), or white lead, was known to the ancients. The Greeks called it [-], or [-], and later [-]. It was employed by Hippocrates¹ in medicine. Theophrastus² describes the method of preparing it by exposing lead to the fumes of vinegar in earthen vessels. Dioscorides³ and Pliny⁴ also notice this mode of procuring it. Besides the preceding, white lead has also been known by other names, such as magistery of lead (magisterium plumbi), flake white, subcarbonate of lead, &c.

Natural History. — Neutral or monocarbonate of lead is found native, both crystallized and massive, in Scotland, England, &c. It is called white lead ore.

Preparation. — Various methods are described for the preparation of carbonate of lead, but the products obtained by some of the processes differ from those procured by others.

1. Neutral carbonate of lead is obtained by adding a solution of an alkaline carbonate to a solution of acetate or nitrate of lead. If acetate of lead and carbonate of potash be employed, the reactions are as follows: — PbO,A+KO,CO₂=KO,A+PbO,CO₂. Procured in this way it is deficient in body, owing to the transparency of the crystalline grains; and it is not, therefore, fitted for the use of the painter, who requires a dense and opaque carbonate. The compounds which possess the required properties expressed by the term body, are basic carbonates, composed of the neutral carbonate and the hydrated oxide.

2. Basic carbonate of lead, called in commerce ceruse, or white lead, is prepared in various ways.

⁵ Cours ds Chimie Générale, t. 2nde, p. 487, 1848.a. By decomposing a subsalt of lead by a stream of carbonic acid. At Clichy, in France, it is obtained by transmitting a stream of carbonic acid through a solution of subacetate of lead. The precipitate, according to Hocbstetter, consists of 2(PbO, CO₂) + PbO,HO. Pelouze and Premy⁵ represents as the neutral carbonate: 3PbO,A+ 2CO₂=2(PbO,CO₂)+PbO,A. The residual neutral acetate of lead is boiled with excess of litharge, and thereby converted into the tribasic acetate, which is used for the preparation of another portion of white lead. A modification of the process has been employed by Messrs Gossage and Benson, of Birmingham. Pinely powdered litharge is moistened, mixed with about 1/100th part of acetate of lead, and de composed, during constant stirring, by carbonic acid. Messrs. Button and Dyer substituted subnitrate of lead for subacetate, and decomposed by car bonic acid. In all the above processes, the carbonic acid used may be obtained by the combustion of coke, or from a lime-kiln.

b. By exposing plates, or bars, or other forms of lead, to the vapour of acetic acid, and, at the same time, to air loaded with carbonic acid gas. In this country white lead is extensively manufactured by the old or Dutch process, which, it is said, yields a product superior as an oil pigment to that obtained by most other methods.

¹ At one manufactory which I inspected, the lead was cast into rectangular gratings on a woodea mould. in the act of cooling the metal underwent crystallisation; a change which I wag informed was essential to the success of the process, as hammered or rolled lead does not so readily becoa* corroded as the crystallised cast metal.

² At the manufactory before alluded to, I was informed that the process required from six to twelve weeks.

³ In some manufactories, the white lead is separated by hand from the residual metallic lead.In the Dutch process, introduced into England about 1780, lead is cast into plate or bars, or into the form of stars, or circular gratings of six or eight inches in diameter, and from a quarter to half an inch in thickness:¹ five or six of these are placed one above another in the upper part of a conical earthen vessel, something like a garden-pot, us the bottom of which there is a little strong acetic acid. The pots are then arranged suitby side, on the floor of an oblong brick chamber, and are imbedded in a mixture of new and spent tan (ground oak bark as used in the tan-yard). The first layer of pots is then covered with loose planks, and a second range of pots imbedded in tan is placed upon the former; and thus a stack is built up so as entirely to fill the chamber with alternate ranges of the pots containing the lead and acetic acid, surrounded by, and imbedded in, the tan. Several ranges of these stacks occupy each side of a covered building, each stack containing about 12,000 of the pots, ana from 50 to 60 tons of lead. Soon after the stack is built up the tan gradually heats or ferments, and begins to exhale vapour, the temperature of the inner part of the stack rising to 140° or 150°, or even higher. The acetic acid is slowly volatilised, and its vapour passing readily through the gratings or folds of lead, gradually corrodes the surface of the metal, upon which a crust of suhact tate is successively formed and converted into carbouate, there beiug an abundant suppr. of carbonic acid furnished by the slow fermentative decomposition of the tanners' bark. In the course of from four to five weeks² the process is completed, and now, on unpacking the stacks, the lead is found to have undergone a remarkable change: the form of the castings is retained, but they are converted, with considerable increase of bulk, into dense masses of carbonate of lead: this conversion is sometimes entire, at others it penetrates only to a certain depth, leaving a cential skeleton as it were of metallic lead, the conversion being uuequal in different parts of the stack, and varying in its perfection at different seasons, temperature, and states of the atmosphere. The stacks are so managni that they are successively being built up and unpacked. The corroded and converted gratings or cakes are then passed through rollers,³ by which the carbonate of lead (white lead) is crushed and broken up, and the central core of metallic lead (blue lead), if any remain, is easily separated: the white lead is then transferred to the mills, where it ts ground up into a thin paste with water, and is ultimately reduced, by the process of elatriation or successive washings and subsidences, to the state of an impalpable powder; it is then dried in wooden bowls, placed upon shelves in a highly heated stove, and thus brought to the state of masses easily rubbed between the fingers iuto a fine powder, in which the microscope does not enable us to discern the slightest traces of crystalline character (Brande)."

"If intended for the use of the painter, it is next submitted 1o grinding with linseed oil; and it is found that a hundred- weight of this white lead is formed into a proper consistence with 8 pounds of oil, whereas precipitated white lead requires 16 pounds of oil for the same purpose; the oue covering the surface so much more perfectly, and having so much more body than the other."

"It is sometimes supposed in this process that the oxygen and carbonic acid reqnind to form the carbonate of oxide of lead are derived from the decomposition of the acetic acid; but this is evidently not the case, for not more thau 100 pounds of real acetic acid exist in the whole quantity of the diluted acid contained in the several pots of each stack; and in 100 pounds of acetic acid there are not more than 47 to 48 pounds of carbon, whereas 6740 pounds would be required to furnish the carbonic acid which should convert 50 tons of lead (the average weight of that metal in each stack) into carbonate of lead. There can be no doubt, then, that the carbon or carbonic acid must come from the tan, and that the oxygen is partly derived from the same source, and partly from the atmosphere: the principal action of the acetic acid therefore is to form successive portions of subacctate of lead, which are successively decomposed by the carbonic acid: the action is, however, of a very remarkable description, for even masses of lead, such as blocks of an inch or more in thickness, are thus gradually converted through and through into carbonate, so that if due time is allowed there is no central remnant of metallic lead. The original texture of the lead is much concerned in the extent and rapidity of the conversion. Rolled or sheet lead will not answer, and the gratings, coils, and stars, which are employed, are all of cast-lead. The purest metal is also required, for if it contains iron, the resulting white lead acquires a tawny hue, and if a trace of silver, it acquires a perceptible dinginess when it is subjected to the action of light" (Brande).

¹ Pharmaceutical Journal, 1849-50, p. 410.[A patent was taken out in 1849, by Mr. J. E. Rodgers,¹ for an improved process of making white lead. The principal features of this process are, first, the use of a chamber capable of being made air-tight, when required, and into which the supply of carbonic acid gas, acetic acid, or aqueous vapours may be controlled or regulated; secondly, the introduction of steam into the converting chamber, either alone or combined. Pieces of lead are suspended in this chamber over troughs containing a fluid in a state of viscous or acetous fermentation. Steam-pipes from a boiler open into this chamber so as to introduce aqueous vapour about three times in twenty-four hours. — Ed.]

The presence of copper in the lead alters the appearance of the resulting white lead. A minute quantity of lamp black, indigo, or some other blue pigment, is usually added to white lead at the time of grinding it with oil, to destroy the yellow tint. By conversion into white lead, the metal usually jains an increase in weight of about a fourth part.

3. By the mutual action of lead, water, and carbonic acid, a hydro-carbonate of lead is produced. The crust formed in leaden cisterns and pipes, by the action of a pure water and air containing carbonic acid, is of this kind (see p. 292). [It is a crystalline compound of carbonate and hydrated oxide of lead, 2(PbO,CO₂) + PbO,HO. — Ed.]

Some years ago, a manufactory for white lead, founded on this principle, was established at Pimlico, but it was soon abandoned. Granulated lead was agitated in water, and the resulting hydrated oxide of lead was exposed to the action of carbonic acid (Brande).

Properties. — Both the carbonate and subcarbonates of lead are white, effervesce with nitric acid, are blackened by sulphuretted hydrogen or the liydrosulphurets, give out their carbonic acid, and are converted into the yellow oxide, when heated. By the aid of carbonic acid, they are very slightly soluble in water.

a. Of the neutral carbonate (PbO,CO₂). — The native carbonate is found crystallised in forms derived from a right rhombic prism. The artificial carbonate is a white amorphous powder.

b. Of the subcarbonates. — These are compounds of the neutral carbonate with the hydrated oxide. They are dull, white, and dense. They possess great covering power, or body. The dicarbonate (2PbO, CO₂,HO), however, does not, according to Bonsdorff, possess the covering power of common white lead. The smaller the quantity of hydrated oxide present, the less readily does the subcarbonate become yellow or brownish by exposure to air and light. Examined by the microscope, none of the subcarbonates present any traces of crystalline texture.

Characteristics. — Heated on charcoal by the blowpipe flame, the carbonates of lead yield soft malleable globules of metallic lead, usually surrounded by a small quantity of the yellow oxide (PbO). Sulphuretted hydrogen and the liydrosulphurets blacken them (PbS). They dissolve in nitric acid with effervescence: the characters of the nitric solution have been already stated. The neutral carbonate and subcarbonate are distinguished from each other by quantitative analysis.

Composition. — All these compounds contain carbonic acid (CO₂) combiiwi with oxide of lead (PbO). Some of them also contain the hydrated oxide (PbO,HO). The proportion of carbonate to hydrate appears to be very different in different preparations.

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Purity. — Carbonate of lead of commerce is rarely pure. It is usually adulterated with earthy or metallic sulphates (as of lime, baryta, or lead). These are detected by their insolubility in diluted nitric acid. Chalk (which is by some used to adulterate it) may be detected as follows: — Dissolve the suspected substance in nitric acid, and precipitate the lead by hydrosulphuric acid. Boil and filter the solution, in which will be contained nitrate of lime (if chalk had been present), recognisable by oxalic acid or oxalate of am monia. Minute portions of lamp black, indigo, or Prussian blue, are added to improve the whiteness of the commercial article. Sometimes a minute portion of the hexacetate of lead (6PbO,A) is found in commercial white lead.

It does not lose weight at a temperature of 212°: 68 grains are entirely dissolved in 150 minims of acetic acid diluted with a fluidounce of distilled water; and the solution is not entirely precipitated by a solution of 60 grs. of phosphate of soda. — Ph. Ed.

Physiological Effects. — Its local effects are not very powerful. Applied to ulcerated surfaces, it acts as a desiccating and astringent substance: swal lowed in large quantities, it does not act as a local irritant, like the acetate. Its constitutional effects are similar to those of the other preparations of lead already described. It appears (see ante, p. 784etseq.) that carbonate of lead more frequently produces colic than the acetate of lead — a circumstance which Dr. Christison thinks may be owing to the great obstinacy with which its impalpable powder adheres to moist membranous surfaces, and the consequent greater certainty of its ultimate absorption.

Uses. — It is never administered internally.

¹ Richter, Ausf. Arzueim. iv. 613.

² Journ. de Pharm. u. 603.Externally it is employed as a dusting powder in excoriations of children and lusty persons; but the practice is objectionable, on account of the danger of absorption. In one case, related by Kopp,¹ a child was destroyed by it. An ointment or plaster of carbonate of lead has been known to give relief in some cases of neuralgia.²

Unguentum plumbi carbonatis, E. D.; Ointment of Carbonate of Lead. (Simple Ointment, ξv.; Carbonate of Lead, ξj. E. Carbonate of Lead, reduced to very fine powder, ξiij.; Ointment of White Wax, lb. j. D. Melt the ointment with a gentle heat, then add the carbonate of lead gradually, and stir the mixture constantly until it concretes, D.) — This ointment is valuable as a cooling and desiccating application to excoriated surfaces or burns.